As a result of ever-increasing environmental obligations and cost pressure, there is at present a trend toward preparing PSAs with little if any solvent. This objective can most easily be realized by means of the hotmelt technology. A further advantage is the reduction in production time: hotmelt lines can laminate adhesives much more quickly to carriers or release paper, and so can save time and money.
However, the hotmelt technology always imposes stringent requirements on the adhesives. For high-grade industrial applications, particular preference is given to polyacrylates, on account of their transparency and stability to weathering.
In order to prepare acrylic hotmelts, conventionally, acrylate monomers are polymerized in solution and the solvent is then removed in the extruder in a concentration process.
For producing compositions of very high shear strength it is very common to polymerize macromonomers, which raise the internal cohesion of the PSA by means, for example, of phase separation. For example, macromonomers were polymerized for the first time in U.S. Pat. No. 3,786,116.
U.S. Pat. No. 4,551,388 describes polystyrenes which are terminated with a methacrylate function and are polymerized with various acrylate comonomers. These polymers are used as PSAs. A short time later, macromonomers of this kind were commercialized by Sartomer and sold under the trade name Chemlink® 4500. In the technical bullentin from Sartomer, for example the copolymerization of Chemlink® 4500 with various acrylate monomers to prepare acrylic hotmelt PSAs was recommended as far back as 1984. In U.S. Pat. No. 4,554,324 these macromonomers were used to prepare high-shear-strength acrylic PSAs.
A further important property for the coating of acrylate compositions from the melt is the phenomenon of orientation of the polymer chains, especially for polyacrylates of relatively high molecular mass. As a result of the orientation the polymers in question may have particular properties, which are generally manifested in at least planar anisotropy of the properties.
Some general examples of properties which can be influenced by the degree of orientation in the polymers and/or in the plastics produced from them are their strength or stiffness, thermal conductivity, thermal stability, and anisotropy in respect of permeability to gases and liquids (cf., for example, I. M. Ward, Structure and Properties of Oriented Polymers, 2nd ed. 1997, Kluwer, Dortrecht).
The abovementioned orientation of the macromolecules likewise plays an important part for the properties of PSAs that are relevant to their adhesion; for instance, interesting properties have likewise been found for oriented PSAs.
The generation of a partial orientation in partially crystalline, rubber-based PSAs was described back in U.S. Pat. No. 5,866,249. As a result of the anisotropic adhesion properties it was possible to define innovative PSA applications.
DE 100 34 069.5 describes a method of orienting acrylic hotmelts in which the acrylic PSAs described were crosslinked with actinic radiation on a roller shortly after having been coated from the die.
DE 100 52 955.0 described an application-specific advantage of these oriented acrylic hotmelts. There it was found that oriented acrylic hotmelts, in the form of PSA tapes, possess in particular improved diecutting properties.
A disadvantage in these methods described above is that the orientation slowly decreases over a prolonged period of time, as a result of structural relaxation.
It is an object of the invention, therefore, to provide oriented acrylic pressure sensitive adhesives in which the orientation is maintained over a prolonged period, and which thus do not have the disadvantage of structural relaxation, or have it only to a reduced extent, while not losing the application-relevant advantages of acrylic PSAs. A further object is to provide a process for preparing such oriented pressure sensitive adhesives.